In continuous casting of steel for casting a billet, slab or bloom, molten steel is poured from a ladle to a tundish and from the tundish to a cooled mould where the metal begins to solidify and continuously is withdrawn from the mould. The tap hole consists of a ceramic nozzle and the steel flows from the nozzle through a discharge tube that extends into the molten steel in the mould. The discharge tube keeps the flow together and protects it from the oxygen in the air. The outlet flow through the nozzle is usually controlled by means of a vertically movable stopper rod. SE 528543 discloses such a stopper rod arrangement that has means for rotating the stopper rod. The rotation of the stopper rod makes the molten steel rotate, and as a result, the impurities such as oxides and nitrides gather in the center of the outlet flow and will not contact the walls of the discharge tube. In this way, the risk of getting deposits on the walls of the mould is reduced as is the risk of orifice clogging. The nitrides and oxides that reach the mould will float as a slag layer on top of the molten metal or are drawn into the molten metal.